Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2001-02-09
2004-02-24
Jackson, Jerome (Department: 2815)
Coherent light generators
Particular beam control device
Modulation
C372S023000, C372S012000, C372S028000, C372S031000
Reexamination Certificate
active
06697394
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a directly modulatable laser comprising a laser medium inside a laser cavity.
The arrangement is used for direct modulation of the emission of a laser, particularly a solid state laser or fiber laser or a reamplified solid state laser or a reamplified fiber laser.
b) Description of the Related Art
Fiber lasers are known, for example, from R. G. Smith,
Appl. Opt.
11, 2489 (772), H. Po, et al. ‘High Power Neodymium-doped Single Transverse Mode Fibre Laser’,
Electronics Letters,
Vol. 29, No. 17, p 1500 (793) and P. Urquhart ‘Review of rare earth doped fibre lasers and amplifiers’,
IEE Proceedings,
Vol. 135, Pr. J, No. 6, December (788). In order to modulate the light emission of a fiber laser, it is possible to modulate the light output of the pump light source. However, with this method only modulation frequencies below the relaxation frequency of the fiber laser can be achieved, namely, in general, only several tens to hundreds of kilohertz. Modulation can be improved by continuous pumping of the fiber laser and reamplification of a rapidly modulatable external signal source at the emission wavelength of the fiber laser (seed) in the laser fiber.
The rapidly modulatable signal source can be effected from a laser diode modulated by the injection flow, a Q-switched laser or continuous-wave laser with intensity modulators in the light path between the signal light source and the fiber laser. However, this method leads to low modulation depths because the amplified spontaneous emission (ASE) of the fiber laser leads to a reduction in contrast when the signal light source is switched off. Further, a temporary exaggerated or elevated power of the output signal occurs when the signal light source is switched on after a long dark period because of the elevated inversion in the active fiber which builds up during the dark period.
This can be prevented by a two-wavelength, two-polarization or two-direction method described in the German Patent Application DE 198 29 684 A1. The fiber amplifier is kept constantly in saturation by switching over or modulating emission light of two wavelengths, two polarizations or two radiating directions through the fiber laser in such a way that the sum of the two light outputs remains constant. In this way, unwanted ASE is completely suppressed and high contrast is achieved. An inversion elevation after dark periods is prevented. The switching over or modulation of the signal light is carried out by modulating the injection flow of two laser diodes serving as seeds.
The disadvantage in this method is that the emission wavelengths of the signal source and fiber laser must match. This is not always possible when using a diode laser as signal source, particularly in the visible spectral range. Further, coupling two laser diodes into a monomode fiber requires complicated opto-mechanical precision elements. It is not possible to provide compact, adjustment-free systems comprising pump light sources, changeover switches or modulators and fiber lasers.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to provide a compact component group as a combination of a solid state laser or fiber laser or reamplified solid state laser or reamplified fiber laser and an internal modulation arrangement which is capable of delivering intensity-modulated laser light up to extremely high modulation frequencies and high light outputs. Further, the invention should solve the problem of insufficient output strength of individual components of modulatable solid state lasers, fiber lasers and amplifiers. Further, a modulation of the light of at least one wavelength should be carried out in such a way that its intensity curve follows the applied electric modulation function more accurately than was possible heretofore.
The invention relates to a directly modulatable laser comprising an active medium inside a laser cavity formed by a resonator mirror and an out-coupling mirror, and a pump light source exciting the active medium.
The invention is characterized, in a first instance, in that the active medium generates radiation of two wavelengths &lgr;
1
and &lgr;
2
and the resonator mirror is constructed as a controllable reflector by which the reflectivity is controllable for each of the two wavelengths &lgr;
1
and &lgr;
2
and the controllable reflector is connected with a control unit, wherein the reflection factor is controlled in such a way that the inversion density of the electrons which is generated in the active medium is constant and the output of one of the wavelengths is controllable between a minimum value and a maximum value according to an applied control signal E, wherein the control of the two wavelengths &lgr;
1
and &lgr;
2
is carried out in push-pull. This arrangement is also known as a directly modulatable laser according to the two-wavelength method.
In a first construction, the two emission wavelengths are generated in a broad-band amplifying medium, wherein these emission wavelengths are selected by the configuration, according to the invention, of the laser resonator or cavity.
In a second construction, the two emission wavelengths are generated in an active medium amplifying on two different lines.
The invention is described in this case only for two wavelengths. Of course, the directly modulatable laser can also be operated with three or more wavelengths, wherein the basic ideas of the invention must be utilized. Accordingly, in this case, the fiber laser is operated with two wavelengths. Because of the wavelength-selective characteristics of the reflectors, the resonance condition in the laser is given only for the wavelengths that are predetermined by the construction of the reflectors. The laser accordingly emits only on these two wavelengths. The amplification factor is adjusted by means of controlling the reflection factor, this control being independent for both wavelengths. In general, the control can be carried out in such a way that the sum of the emission outputs of the two wavelengths is constant.
The controllable reflector is used in this case as a wavelength-selecting resonator mirror of the laser or of a reamplified laser. The controllable reflector contains at least one modulator for the phase position and/or the polarization and/or the optical power of two light components. The effect or action of an intensity modulation for the two emission wavelengths or for only one of these wavelengths can be utilized at the output of the laser or reamplified laser.
The two emission wavelengths are not taken from external signal sources in this case, but from the active-ion doped laser fibers themselves, whose wavelength spectrum encompasses both emission wavelengths, or an actively doped fiber is used which can emit two discrete wavelengths. The amounts of the two emission wavelengths are determined in the two-wavelength method by the construction of the wavelength-selecting resonator mirror and the emission spectrum of the active fiber.
In a second instance, the invention is characterized in that the active medium generates radiation of a wavelength with two polarization directions P
1
and P
2
and the resonator mirror is constructed as a controllable reflector by which the reflectivity is controllable for each of the two polarizations P
1
and P
2
and the controllable reflector is connected with a control unit, wherein the control of the reflection factor is carried out in such a way that the inversion density of the electrons which is generated in the active medium is constant and the output of one of the polarizations is controllable between a minimum value and a maximum value according to an applied control signal, wherein the control of the two polarization directions P
1
and P
2
is carried out in push-pull.
This arrangement is also known as a directly modulatable laser according to the two-polarization method.
The controllable reflector is used in this case as a polarization direction-selecting resonator mirror of the laser
Ruske Jens-Peter
Tuennermann Andreas
Zellmer Holger
Jackson Jerome
Jenoptik LDT GmbH
Landau Matthew C.
Reed Smith LLP
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